Edge bonding is widely employed in composite structures, such as honeycomb core sandwich panels or grid stiffened shells suitable for various applications, such as those used in the aerospace industry. In moderate and lower temperature range applications, the elements of the structures may be formed from polymeric matrix composites reinforced with various types of fibers. Core elements or stiffeners are joined to facesheets or skins using edge bonds. For example, as shown in FIG. 1, a polymeric matrix composite structure 20 includes spaced facesheets 22, 24 joined together by web-like stiffeners 26 that are edge bonded at 27 to the facesheets 22, 24. Since the stiffeners 26 are required to transmit compressive, tensile and shear loads, it may be necessary to reinforce the edge bonded joints 27 using fillets 28 which may comprise a polymeric adhesive. The fillets 28 adhere to both the facesheets 22, 24 and the sides of the stiffeners 26, covering the bond line (not shown) at the edge bonded joints 27. The structure 20 shown in FIG. 1 is well suited for forming honeycomb sandwich structures that distribute loads to peripheral members or carry in-plane loads while resisting buckling. Moreover these structures are relatively economical to fabricate, due in part to the simplicity of the edge bonded joints 27.
In higher temperature applications however, such as those in which parts are subjected to high temperature exhaust gasses and/or atmospheric friction during re-entry of space vehicles, the polymeric-based construction shown in FIG. 1 is not suitable, due to the fact that polymeric matrix materials lose strength and stiffness at high temperatures.
In higher temperature applications, such as the aerospace applications mentioned above, ceramic matrix composites (CMC) are commonly used because they are better suited to withstanding higher operating temperatures. Typically, CMC's comprise multiple layers of fiber fabric impregnated with a matrix material, often referred to as a prepreg, although ceramic fibers are available in a variety of other forms, including tape, tows and three dimensional woven preforms. Several methods may be used to fabricate CMC's, including for example, hand-layup, filament winding, tow placement and dry fiber preforms with vacuum assisted matrix infusion. The construction shown in FIG. 1 however, is not directly reproducible using CMC materials because the production of CMC's involves a sintering step in which unfilled ceramic matrix material, such as that in fillets, tends to sublimate. Prior attempts to make honeycomb sandwich structures using CMC's have left a ceramic matrix bondline only in the small area where the edges of the honeycomb form a faying surface with the adjacent facesheets. The resulting bond is may be weak, even at very low loads.
Accordingly, there is a need for CMC structures employing edge bonded joints having improved joint strength in order to carry heavier loads. There is also a need for CMC structures having edge bonded joints that can be easily and economically fabricated. The disclosed embodiments are intended to satisfy these needs.